This invention relates to a so-called `switch mode` power converter device of the kind wherein a D.C. voltage supply is provided by converting an input voltage derived from an A.C. or D.C. voltage source.
For sensitive equipment which requires minimisation of noise in order to achieve reasonable performance a suitably designed linear supply is usually employed. The use of linear supplies, however, is precluded when space, weight or efficiency are at a premium or when only a D.C. input voltage is available as a power source. In such cases, it is unnecessary to utilise a switching supply technique. Conventional switch-mode power supplies suffer, however, from noise problems that arise from the rapid energy transfer which is necessary to maintain efficiency.
With a view to reducing fast current edges occurring in a switch-mode power supply, it is known to utlise in the power output stage of a switching converter, a series resonant circuit. A power converter of this type is known, for example, from `High frequency series resonant power supply--Design Review`; Unitrode Power Supply Design Seminar Book 1986; Pg A2-1. This publication describes a power converter for transforming an input voltage to provide a D.C. voltage output and comprising transformer means having an input winding coupled to a voltage input circuit and an output winding coupled to a rectifier circuit for providing said D.C. voltage output, said voltage input circuit including a voltage switching means for switching an input voltage at a given frequency and a resonant circuit so coupled to said voltage switching means that operation thereof is effective to drive current pulses of approximately sinusoidal wave-form through the said input winding.
Although such a known arrangement is effective in reducing electromagnetic interference in comparison with a conventional switch-mode power supply, difficulties do arise because of electrostatic interference transferred between the input and output circuits via the capacitive coupling both in the voltage transformer and the other circuit elements.